During the past several years, it has been demonstrated that enzyme-catalyzed reactions in anhydrous, low polarity organic solvents represent a valuable addition to organic chemistry. Various reactions employing enzymes as catalysts have been disclosed, including esterifications, transesterifications, aminolyses and lactonizations. Except for naturally occurring polyesters such as poly(beta-hydroxybutyrate), there appears to have been limited previous effort to prepare polyesters using enzymes as catalysts. See, for example, Okumura et al, Agric. Biol. Chem., 48, 2805 (1984); Ajima et al, Biotechnology Lett., 7, 303 (1985); Matsumura et al, Makromol. Chem., Rapid Commun., 7, 369 (1986); Kitazume et al, Chem. Express, 3, 135 (1988); and Margolin et al, Tetrahedron Letters, 28, 1607 (1987). For example, Okumura et al describe the Aspergillus niger lipase catalyzed oligomerization of 1,2-ethanediol and 1,3-propanediol with the diacids from 1,6-hexanedioic acid through 1,14-tetradecanedioic acid using either excess diol or excess diol with a small amount of water added as the solvent system. The only products examined in detail proved to be a "trimer", a "pentamer" and a "heptamer" of the forms AA-BB-AA, AA-BB-AA-BB-AA, and AA-BB-AA-BB-AA-BB-AA, respectively, which formed from 1,3-propanediol (AA) and 1,13-tridecanedioic acid (BB) in a ratio of 1:8:4.5 after 24 hours. The words "dimer", "trimer", etc. have been placed in quotes to reflect that they are being used to indicate the total number of monomer units in the oligomer rather than the number of repeat units. Thus, the "dimer" is really one repeat unit, the "trimer" is really 1.5 repeat units, the "pentamer" is really 2.5 repeat units, etc. Separation of the higher oligomers from the reaction mixture seemed to limit the degree of polymerization possible, but, at the same time, protected the oligomers from enzymatic hydrolysis or transesterification by the large excess of diol present. Apparently the "heptamer" is either too insoluble to have a favorable rate of conversion to higher oligomers or its rate for acylating the enzyme at other than a terminal ester is rapid, and it is converted back to lower oligomers.
Ajima et al described the first attempted enzyme-catalyzed polymerization of an A-B type monomer, 10-hydroxydecanoic acid. The reaction was performed in benzene using a poly(ethyleneglycol) solubilized lipoprotein lipase from Pseudomonas fluorescens. The product was found to have a longer retention on gel permeation chromatography (GPC) than did the monomer suggesting a structure having a smaller molecular volume. In the absence of data to the contrary, it seems likely that the product was a lactone or, possibly, a bislactone (dilide) rather than the proposed oligomeric material.
Matsumura et al also described attempted polymerizations of .omega.-hydroxycarboxylic acids in water and in organic solvents using lipases from Candida rugosa and Chromobacterium viscosum as the catalysts. The substrates chosen were the primary alcohols 12-hydroxydodecanoic acid and 16-hydroxyhexadecanoic acid and the secondary alcohols 12-hydroxyhexadecanoic acid and 12-hydroxy-cis-9-octadecenoic acid. While most of the substrate was consumed, the products, even with primary alcohols, were principally trimers and tetramers.
Kitazume et al described a second polymerization of an A-B monomer. A prochiral fluorine-substituted diester was enantioselectively hydrolyzed to the corresponding half-ester with lipase from the yeast Candida cylindracea. The resulting carboxylic acid was coupled nonenzymatically with the amine in a t-butyldimethylsilyloxy aniline or the unprotected amine in N-acetyl-1,4-benzenediamine to provide, after deprotection and hydrolysis of the remaining ester, an hydroxy acid or an amino acid A-B monomer. The monomer was polymerized using a modified cellulase from Trichoderma viride in benzene, hexane, or Cl.sub.2 CFCF.sub.2 Cl. Molecular weights (Mw) as high as 18,500 daltons and low polydispersities (1.09-1.51) were found for the polyester and polyamide products. However, the source of this example's success, in view of the very limited success of the related reactions just described, is unclear.
Margolin et al have described the stereoselective reaction of bis(2-chloroethyl)(.+-.)-2,5-dibromoadipate with 1,6-hexanediol, the reaction of bis(2,2,2-trichloroethyl)(.+-.)-3-methyladipate with 1,6-hexanediol, and the reaction of bis(2-chloroethyl) adipate with (.+-.)-2,4-pentanediol in toluene using commercially available lipases as catalysts. In each case, the reaction was allowed to continue for an extended period and provided, principally, a mixture of a "trimer" and a "pentamer" of the forms AA-BB-AA and AA-BB-AA-BB-AA where AA represents the diol and BB represents the diacid moiety of the oligomer.